This invention relates to RF load and source pull testing of low noise as well as medium and high power RF transistors and amplifiers at millimeter-wave frequencies.
Modern design of low noise or high power RF amplifiers and mixers, used in various communication systems, requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is insufficient for the transistors, which operate either as very low noise or as highly non-linear devices, close to power saturation, to be described using linear or non-linear numeric models.
A popular method for testing and characterizing such microwave components (transistors) in the non-linear region of operation is “load pull” or “source pull” (see ref. 1). Load/source pull is a measurement technique employing microwave impedance tuners and other microwave test equipment (FIG. 1), such as signal source, source (input) and load (output) tuner, input and output power meter and test fixture which holds the DUT. The tuners and equipment are controlled by a computer via digital communication cables. The microwave impedance tuners are devices which allow manipulating the RF impedance presented to the Device Under Test (DUT, or transistor) to test (see ref. 1); this document refers hence to “impedance tuners”, (see ref. 2), in order to make a clear distinction from “tuned receivers (radios)”, popularly called elsewhere also “tuners” because of the included frequency tuning circuits (impedance tuners versus frequency tuners).
Electro-mechanical impedance tuners (FIGS. 2 and 3) in the microwave and low millimeter-wave frequency range typically between 100 MHz and 60 GHz are using the slide-screw concept: it comprises a parallel plate low loss airline (slabline, (24), (30) with a test port (36) and an idle port (37), a center conductor (23, 31) and one or more mobile carriages (34) which carry a motor (35), a vertical axis (38), which controls the vertical position of a reflective probe (32), see ref. 2. The carriages are moved horizontally by additional motors (not shown) and gear. The signal enters one port (36) and exits from the other (37). In load pull the test port is the one where the signal enters, in source pull the test port is the one where the signal exits, FIG. 3 shows a load tuner. The entire mechanism is, typically, integrated in a solid housing (33) since mechanical precision is of highest importance. Millimeter-waves occupy the frequency spectrum from approximately 30 GHz to 300 GHz. The wavelength (λ) is in the 10 mm to 1 mm range. The wavelength is calculated as: λ[mm]=300/frequency[GHz]. The tuners in this invention operate from 20 to 110 GHz, corresponding to wavelengths between 15 and 2.72 mm A slide screw tuner must allow horizontal travel of the tuning probe of one half of a wavelength, or, in the case of the lowest frequency (20 GHz), at least 7.5 mm, in order to allow for a 360° rotation of the created reflection factor (the reflection phase is double the transmission phase).
The typical configuration of the reflective probe inside the slabline is shown in FIG. 2: a parallel reflective tuning element (21) also called “tuning” probes or slug, is inserted into the slotted transmission airline (24) and coupled capacitively with the center conductor (23) to an adjustable degree, depending from very weak (when the probe is withdrawn) to very strong (when the probe is very close (within electric discharge—or Corona) to the center conductor; it must be pointed that capacitive “tuning” probes are different from electro-magnetically coupled “sampling” probes, which are loosely coupled with the center conductor; when the tuning probes move vertically (26) between a “top position” and a “bottom position” and approach the center conductor (23) of the slabline (24) and moved along the axis (25) of the slabline, they alter the amplitude and phase of the reflection factors seen at the slabline ports, covering parts or the totality of the Smith chart (the normalized reflection factor area). The relation between reflection factor and impedance is given by GAMMA=(Z−Zo)/(Z+Zo), where Z is the complex impedance Z=R+jX and Zo is the characteristic impedance. A typical value used for Zo is 50Ω (Ohm) (see ref. 3). In a 50Ω test system (i.e. when the tuner is terminated at both ports with 50Ω), GAMMA is equal to the first element of the tuner twoport s-parameter matrix: GAMMA=S11, also expressed as Standing Wave Voltage Ratio VSWR (see ref. 3).
Up to now such metallic tuning probes (slugs) (FIG. 2) have been made in a cubical form (21) with a concave bottom, which allows capturing, when approaching the center conductor (23), the electric field on the sides, where it is concentrated in the closest space between the center conductor and the ground planes of the slabline. This field capturing allows creating high and controllable reflection factors. The compact millimeter-wave tuner disclosed here uses rotating probes (see ref. 7); these probes can be circular and rotate eccentrically, or can be oval, or elliptical and rotate centered or eccentrically (FIG. 6). The important feature is for the coupling factor (or the penetration of the disc body into the slabline) to be controllable only through the angle of rotation. This allows avoiding cumbersome vertical axis structures (38) in FIG. 3).
The interconnections between tuning probe inside the tuner and the wafer-probe introduce, especially at millimeter-wave frequencies, insertion loss (FIGS. 1, 8 (item 87) and 10B), which reduces the tuning range of the tuner and by consequence the capability of the tuner to conjugate match many transistors (FIG. 4). It is therefore of primary importance to reduce or eliminate the interconnections. The new compact tuner allows doing this, because it can be mounted, at the appropriate angle, directly on the wafer-probe.